Optical glasses or optical transparent resins are used as materials for optical elements used for optical systems of various cameras such as cameras, film integrated cameras, and video cameras. There are various types of optical glasses which have excellent properties such as heat resistance, transparency, dimensional stability, chemical resistance, etc. and varieties of refractive indices and Abbe numbers, but such optical glasses have problems of poor forming fabricability and low productivity, in addition to high material cost. In particular, processing into aspherical lenses that are used for aberration correction requires an exceptionally high level of technique and high cost, which are therefore serious obstacles for practical use.
In contrast to the aforementioned optical glasses, optical transparent resins, particularly, optical lenses consisting of thermoplastic transparent resins have advantages that such optical lenses can be mass-produced by injection molding, and aspherical lenses also can be easily produced, and are therefore used currently as camera lenses. Examples of the optical transparent resins include polycarbonate consisting of bisphenol A, polymethyl methacrylate, or amorphous polyolefin. Further, polycarbonate resins are particularly used also as sheets or films for optical applications. Sheets and films consisting of polycarbonate resins have high transparency and heat resistance, and are therefore used suitably for front protective sheets, light guiding sheets, or the like, of liquid crystal display devices.
However, polycarbonate resins consisting of bisphenol A have a disadvantage of high birefringence, and therefore the applications thereof are constrained. In particular, in applications for mobile phone cameras and digital cameras in recent years, there is a growing demand for resin materials having high imaging performance and low birefringence, with an increase in resolution due to an increase in the number of pixels. Patent Literature 1 discloses that use of dicarboxylic acid having a fluorene structure as a raw material for polyester resins is effective for reducing the birefringence.
Aiming at further excellent materials, a resin having various excellent optical properties such as high refractive index and low Abbe number has been developed (Patent Literature 2). However, in recent years, while electronic devices such as digital cameras, smartphones, and tablets are widely adopted, and various models are put on the market, functions of cameras mounted on such devices are being progressively enhanced (such as higher pixel density and lower F value). Therefore, lenses having high refractive index and small size are required, and aspherical lenses are often used in designing lens units. Further, with the enhancement of functions of the devices, sheets or films having excellent shapability are also desired. In order to obtain such precision lenses and sheets or films having excellent shapability, resins having not only optical properties but also good fluidity and strength are desired.